High-temperature reactors are very safe compared with other reactor systems, and a secondary shut-down system is not always required.

In the Peach Bottom reactor safety fuses are built in the upper part of the reflector. From these fuses hang short absorber rods, which fall into the core if the melting temperature of these fuses is exceeded.

In the Fort St Vrain reactor it is possible to compensate up to 12 nile of reactivity introducing B4C in granular form in the core channels.

Other secondary shut-down systems can also be provided by poison gases. The introduction of 1 atm of N2 or Kr can produce a reactivity loss of the order of 1 — f — 3% depending on the voidage (Kr is absorbing approximatively 1.5 times as much as N2). In the THTR reactor up to 14 nile reactivity might be compensated injecting BF3 as poison gas for additional emergency shut-down. The difficulty posed by cleaning the primary circuit before restarting the reactor discourages the use of poison gases.

Reference

1. P. Marien, Research and Development Division First Semi-annual Report for the period 1st July 1960-31st December 1960, p. 63. Dragon Project Report 34.